TDD systems include the flexibility of bandwidth allocation in an unpaired frequency band, and the flexibility of choice on a downlink to uplink resource allocation ratio (referred to as “D/U ratio” herein). The latter is attractive because of the emerging traffic service types and traffic volume turbulence, both of which result in a wide range of D/U ratios. On the other hand, the requirement for system-wide synchronization is traditionally a major disadvantage of TDD systems. Under this requirement, all base stations or all user equipment must follow the same system timing to turn off a transmitter, in order to avoid overlapping between downlink and uplink signals in the overall system.
The timing requirement may weaken TDD features regarding D/U ratio flexibility. First, because all base stations and user equipments are synchronized, there can be only one D/U ratio per carrier frequency, on a system-wide basis. Second, once the D/U ratio is determined for a system, it may be too expensive to change it to other values because, before synchronously changing the D/U ratio, all transmitters have to shut off the transmission all together. The costs paid for such a “cold restart” include a huge loss of system capacity and interruption of user traffic. In addition, the monitoring and management of unfinished traffic in the entire system is expensive and time-consuming.
One previous invention provides a systematic method to accomplish two targets:                i. Live-change (without cold restart) of the D/U ratio system-widely from one value to another; and        ii. Live-change (without cold restart) of the D/U ratio within a specific deployment area to a different value from one in surrounding areas.        
In some LTE TDD systems, exemplary downlink-uplink allocations are specified in Table 1.
TABLE 1Downlink-Uplink Allocations in LTE-TDDCon-Switch-pointSubframe numberfigurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
The subframes in LTE-TDD can be downlink subframes (D), uplink subframes (U) and special subframes (S) that includes three fields Downlink Pilot Timeslot (DwPTS), Guard Period (GP) and Uplink Pilot Timeslot (UpPTS).
It is suggested by a previous invention to specify a fourth subframe type: a mute subframe M. The mute subframe M is a special subframe that has neither a downlink signal nor an uplink signal during its full subframe duration. If the system plans to convert a certain downlink subframe to uplink or vice versa, it has to mark the subframe as a mute subframe M first. Once the downlink or uplink subframe is marked as a mute subframe M, it shall not be used for any transmission until it is marked as either a downlink subframe or an uplink subframe again. The system can assign the mute subframe that is originally a downlink (or uplink) subframe to be used as an uplink (or downlink) subframe. With the creation of a mute subframe M, the network could change the D/U ratio either system-wide or only for certain deployment areas.
The D/U allocation change has some logic impact to existing Hybrid Automatic Repeat-request (HARQ) timing specifications in LTE-TDD systems. HARQ is the process by which the traffic transmission is acknowledged by the receiver end, which sends ACK/NAK signaling on an opposite communication link, such that traffic retransmission may be triggered upon negative acknowledgement (NAK). The delay between traffic (re)transmission and acknowledge feedback (ACK/NAK) is predetermined. In addition, the delay between negative acknowledgement (NAK) and retransmission is also predetermined on the LTE uplink.
According to LTE specifications, user equipment (UE) shall transmit ACK/NAK in uplink subframe nU for traffic transmissions on physical downlink shared channel (PDSCH) in subframe nU−k, where kεK and K (defined in Table 2) is called a downlink association index set of M elements {k0, k1, L kM-1} depending on the subframe nU and the UL-DL configuration. TDD ACK/NAK bundling and multiplexing is performed by a logical AND operation of all individual ACK/NAKs corresponding to HARQ packets across multiple downlink subframes (ACK/NAK bundling) or HARQ packets across single downlink subframe.
According to LTE specifications, the UE shall transmit a new data packet or re-transmit an old data packet on a physical uplink shared channel (PUSCH) in subframe nD+kPUSCH upon a scheduling command or negative ACK/NAK in downlink subframe nD; the UE shall expect ACK/NAK on a physical HARQ indication channel (PHICH) in downlink subframe nU+kPHICH for its traffic (re)transmission on PUSCH in subframe nU. kPUSCH and kPHICH are defined in Table 3.
TABLE 2DL HARQ timing in LTEUL-DLACK/NAK Subframe nU: PDSCH in subframe nU − kallocations0123456789C0——64——64C1——7, 64———7, 64—C2——8, 7, 4, 6————8, 7,——4, 6C3——7, 6, 116, 55, 4—————C4——12, 8, 7,6, 5,——————114, 7C5——TBD———————C6——775——77—
TABLE 3UL HARQ timing in LTEkPUSCH for DL subframekPHICH for UL subframenumber nD:number nU:TDD(PUSCH in subframe nD +(PHICH in subframe nU +UL/DLkPUSCH)kPHICH)allocations01234567890123456789C046———46—————476——476C16——46——4——46———46—C2—4—4——6————6——C34———44——666—————C4——44——66——————C5—4——6———————C677———77——5——466——47—